Nitric Oxide Electrochemical Sensors Based on Hybrid Films of Conducting Polymers and Metal Phtalocyanines

Portugaliae Electrochimica Acta 21 (2003) 235-243 PORTUGALIAE ELECTROCHIMICA ACTA Nitric Oxide Electrochemical Sensors Based on Hybrid Films of Conducting Polymers and Metal Phtalocyanines M.C. Miras, A. Badano, M.M. Bruno, C. Barbero * Departamento de Química, Universidad Nacional de Rio Cuarto, Agencia Postal No 3, 5800-Rio Cuarto, Argentina. Received 30 December 2002; accepted in revised form 25 February 2003 Abstract Hybrid films of poly(o-aminophenol) and nickel sulfonated phtalocyanine are produced by electrochemical polymerization of o-aminophenol in the presence of the metal complex. The irreversible incorporation of the metal complex into the hybrid films is tested using reflectance infrared spectroscopy. Electrodes modified with poly(oaminophenol) alone and the hybrid film show a higher electrocatalytic activity, for NO oxidation, than the base substrate electrode (glassy carbon). The hybrid films show higher currents and lower oxidation overpotential. The oxidation peak current is linear with concentration up to 200 µM of NO. Keywords: nitric oxide, sensors, poly(o-aminophenol), phtalocyanine, hybrid films. Introduction Nitric Oxide (NO) has acquired great relevance as biological analyte due to its role as cell signalling molecule, involved in the immunity regulation, defence mechanism and neurotransmission[1]. Additionally, NO has a role in the action of nitrite as a food preserver. A very useful method to monitor NO and related molecules consists in the electrochemical measurement of NO oxidation to higher oxides[2]. In such an use, conventional electrode materials (e.g. platinum) show low sensitivity and selectivity. A viable alternative is the use of carbon electrodes modified with electroactive films. The electrocatalysis on the films would allow to increasing both sensitivity and selectivity. The method could be * Corresponding author. E-mail address: M.C. Miras et al. / Portugaliae Electrochimica Acta 21 (2003) 235-243 used to follow the NO concentration in real time (seconds), giving a time resolved information about metabolic functions. Additionally, using ultramicroelectrodes, it is possible to monitor NO concentration inside the cell. While several electrocatalysts have been used, a polymer produced by electroxidation of nickel hydroxiphenylporphyrin has shown clear electroactivity. Development of stable NO sensors is important for its use in clinical therapy [3]. It has been shown that the nickel porphyrine polymer losses Ni in the testing media [4]. On the other hand, phtalocyanines are very stable materials, with electrocatalytic effects towards different systems [5]. Phtalocyanines form complexes with metal atoms more stable than porphyrines and have been used successfully in more aggressive media [6]. Metal phtalocyanines have been incorporated into polyaniline films for oxygen electrocatalysis [7]. In the present communication, we describe the development of NO sensors based on hybrid films of poly(o-aminophenol) and Ni sulfonated phtalocyanine. Poly(o-aminophenol) films can be produced by electrochemical polymerization of o-aminophenol [8]. The films have been used as conductive support of enzymes in biosensors [9] and a low electrocatalytic activity for oxygen reduction has been reported [10]. Experimental All solutions were produced using ultrapure (Millipore) water and analytical quality reagents. 2-aminophenol (Fluka, purum) was twice recrystallized from ethyl acetate, once from benzene and dried under vacuum. All potentials are reported against calomel saturated electrode (SCE) as reference. Nitric oxide production and standardization NO was produced by reaction of nitrite with iodide in acid media. The obtained gas was bubbled trough a 0.1 M KOH solution to retain NO2 and acid droplets. The phosphate buffer (pH=7) used to prepare stock solutions was degassed previously by bubbling N2 for 1 hour. The solutions were stored at 4 ºC. The concentration of NO in stock solutions was evaluated by colorimetry of the azo 236 M.C. Miras et al. / Portugaliae Electrochimica Acta 21 (2003) 235-243 dye formed by coupling of the diazonium salt of sulfanilic with 1-naphtylamine [11]. The diazonium salt is formed by reaction of the sulfanilic acid with NO. During measurements the cell was maintained under a blanket of N2 to avoid oxidation by oxygen of the NO in solution. The peak current of successive voltammograms differs in less than 3% assuring that NO concentration was constant during measurements. Polymerization Poly(o-aminophenol) films were produced by cycling the base electrode (Pt or GC) between -0.25 and 0.7 Vsce at 50 mV/s in a solution 0.05 M of oaminophenol in 0.5 M HClO4. To incorporate the metal complex, sulfonated nickel phtalocyanine (Fluka, puriss) was added to the solution in a 10 mM concentration. Electrochemistry The electrochemical experiments were performed using a GAMRY PC4/750 potentiostat controlled by a personal computer. A conventional three electrode cell was used with a Pt wire as counter electrode and a saturated calomel electrode as reference. The working electrode materials were either GC or Pt. The GC electrodes were GC disks made by pressing a 3 mm diameter glassy carbon rod inside a Teflon tube (2.95 mm internal diameter). The electrode surface was polished with alumina powder (down to 1 µm). The electrode used for infrared reflectance was built with a 0.5 mm thick Pt plate (1 x 1 cm size), mirror polished. Reflection-absorption FTIR spectroscopy Measurements of the FTIR spectra of polymer films onto electrodes were made by ex-situ reflection-absorption. The measurements were carried out in a SpectraTech specular reflectance accessory with an incidence angle of 70o. The polymer films were deposited on polished Pt plates and a polished Pt plate was 237 M.C. Miras et al. / Portugaliae Electrochimica Acta 21 (2003) 235-243 used to record the background spectrum. The measurements were carried out in a Nicolet Impact 400 FTIR spectrophotometer with a resolution of 1 cm-1 and 200 scans. To improve the sensitivity to surface species[12], p-polarized radiation was used. Results and Discussion Cyclic voltammograms, taken during electrochemical deposition of a typical hybrid film, are shown in Fig. 1. An oxidation peak for the monomer (o-aminophenol) could be clearly seen at ca. 0.535 Vsce. After several cycles, a new pair of redox peaks at ca. 0.08 Vsce (anodic) and –0.01 Vsce (cathodic) could be observed. The peak system appears in a potential that corresponds to the redox response of poly(o-aminophenol) [13]. The oxidation peak of the monomer decreases in intensity with cycling, indicating that the growth is not autocatalytic as in the case of polyaniline [14]. On the other hand, the current due to the redox response of the film increases with cycling due to the increase of redox charge during growth of the polymer layer. The polymerization in presence of the metal complex is faster than with oami (...truncated)